1. Field of the Invention
This invention relates to a method for the growth of a semiconductor single crystal by the procedure of bringing a seed crystal into contact with a molten semiconductor held in a quartz crucible and pulling up a rod of semiconductor single crystal from the molten semiconductor. More particularly, this invention relates to a method for the growth of a semiconductor single crystal characterized by conferring a prescribed distribution upon the oxygen concentration in the direction of pull of the semiconductor single crystal by the adjustment of the intensity of the magnetic field applied to the molten semiconductor.
2. Description of the Prior Art
In the growth of a single crystal of silicon by the Czochralski method (CZ pull method), a quartz (SiO.sub.2) crucible is used as the container for holding molten silicon. It has been known that the grown single crystal of silicon contains a fairly large amount of oxygen because the silicon at the melting point is chemically active and, therefore, is suffered to react with the quartz of the crucible and melt it into the molten silicon. When the quartz of the crucible melts into the molten silicon, the trace of impurities present within the crucible proper simultaneously melt out and find their way into the grown single crystal of silicon.
The oxygen in the single crystal of silicon, depending on the concentration and distribution thereof, manifests various effects on the characteristics of a semiconductor device made from the single crystal or on the heat treatment performed on the device during the course of production. When the oxygen concentration is relatively high, for example, the heat treatment induces a stacking fault or produces precipitates of oxygen and exerts adverse effects on the characteristics of the semiconductor device.
Incidentally, when the heat treatment is performed with the fault of this nature lodged outside the active region of the semiconductor device, it gives birth to the so-called intrinsic gettering action, namely the action of gettering harmful impurities such as, for example, impurities of such metal as Fe, Cu, and Au, and brings about an effect of improving the characteristics of the semiconductor device.
When the oxygen concentration is high but not to the extent of inducing the occurrence of precipitate of oxygen, it is effective in curbing the occurrence and growth of defects due to oxygen clusters. To be specific, it is capable of repressing the occurrence and growth of defects during the heat treatment performed in the course of the production of the semiconductor device.
This state of affairs has urged the development of techniques for allowing the oxygen in the single crystal of silicon to be distributed at a relatively high level such as, for example, in the range of from 15 to 20 ppma as uniformly throughout the entire length thereof as permissible. Japanese Patent Publication SHO 60(1985)-6,911, for example, discloses a method which, in the step of pulling up a single crystal rod from a molten semiconductor material held in a quartz crucible by the Czochralski method, controls the gradient of the rotational speed profile of the crucible in inverse proportion to that of the axial oxygen concentration of the crystal during the pulling process.
Then, Japanese Patent Application Disclosure SHO 57(1982)-135,796 discloses a method which comprises rotating a seed crystal of silicon grown in size to be pulled up about its axis in the direction opposite that of the rotation of a crucible holding a molten semiconductor at an initial rotational speed greater that of the crucible and increasing the rotational speed of the crucible in accordance as the volume of the molten semiconductor held in the crucible decreases
These methods, however, have the disadvantage that the convection of the molten semiconductor for crystal growth and the vibration of the surface of the molten semiconductor result in conspicuous occurrence of swirly defects and striation of growth and the rotation of the crucible coupled with the convection entails forced current and vortex current within the molten semiconductor and, as a result, the oxygen concentration is varied in a fairly wide range in the direction of the length of pull.
For the purpose of mending this disadvantage, there has been proposed a method for the growth of crystal which, during the operation of pulling up a single crystal from a molten semiconductor while keeping the molten semiconductor under application of a magnetic field and keeping in rotation the container of the molten semiconductor, controls the oxygen concentration in the single crystal by varying the rotational speed of the container [Japanese Patent Application Disclosure SHO 58(1983)-74,594].
This method is capable of curbing the occurrence of swirly striation of growth and effecting the control of oxygen concentration, though to a limited extent.
The attempt at controlling the oxygen concentration by adjusting the rotational speed of the crucible nevertheless entails the disadvantage that the oxygen concentration distribution in the direction of pull of the single crystal is still varied in a fairly wide range. By the control of the revolutionary speed of the crucible, however, accurate control of the oxygen concentration within .+-.5 to 10% in the direction of pull of the single crystal has never been successfully attained.